1. Field of the Invention
The present invention relates to a printer which records a color image on a recording medium, by irradiating the recording medium cyclically with a plurality of colors while feeding the recording medium for use to record the image in a predetermined sub-scanning direction by a motor.
2. Description of the Related Art
Printers of the type described above include a printer which records color images on instant film sheets. While a motor is feeding an instant film sheet in a predetermined sub-scanning direction, the printer records a color image on the instant film sheet by irradiating the instant film sheet in the sub-scanning direction cyclically with emitted colors of red (R), green (G), and blue (B) from light-emitting elements in quantities corresponding to image data based on write command pulses synchronized with rotation of the motor. When recording is performed on an instant film sheet which is being transported, transport speed of the instant film sheet may change due to load changes, and thus some printers are equipped with an illuminating device which detects the transport speed of the instant film by an encoder and performs proper irradiation using an encoder signal outputted from the encoder in case of any change in the transport speed (see, for example, Japanese Patent Application No. 2004-070317).
With the technique disclosed in Japanese Patent Application No. 2004-070317, the instant film sheet is irradiated cyclically with colored lights from light-emitting elements in quantities corresponding to image data by adjusting the shutter speed of liquid crystal shutters arranged in the main scanning direction orthogonal to the sub-scanning direction.
When irradiating an instant film sheet with lights in quantities corresponding to shutter speeds of the liquid crystal shutters, the illuminating device must be equipped with a drive section which changes the liquid crystal shutters from a closed state to an open state, or from an open state to a closed state. By opening and closing the liquid crystal shutters at appropriate shutter speeds using the drive section, it is possible to irradiate an instant film sheet with colored lights in quantities corresponding to image data. Some liquid crystal shutters integrally incorporate a control circuit which opens and closes the liquid crystal shutters (see, for example, Japanese Patent Publication No. 07-9509). Besides, there are various types of liquid crystal, and the drive method of liquid crystals must be changed according to their type to bring out their potential. Thus, to bring out the maximum potential of liquid crystals, ingenuity is exercised in the manner in which signals are applied between electrodes of the liquid crystal shutters by the drive section (such as driving the liquid crystals using two drive signals of different frequencies or driving the liquid crystals with a bias electric field applied) (see, for example, Japanese Patent Publication No. 06-52469, Japanese Patent No. 2503464, and Japanese Patent Publication No. 04-39648).
If voltage of the same polarity is continued to be applied between electrodes of the liquid crystals to open and close the shutters as is done by the drive section according to the techniques described in the patent documents, DC-like electric fields are accumulated in the liquid crystals, and even after the voltage ceases to be applied between the electrodes (bringing about a field-free state), in some cases molecular arrangement in the liquid crystals does not return to its initial field-free state existing before the application of the voltage between the electrodes and is slightly tilted with respect to its original orientation in the initial field-free state.
To deal with this situation, the techniques disclosed in Japanese Patent Application No. 2004-070317 and the like continue to form AC-like electric fields in the liquid crystals by applying voltages of positive and negative polarities between electrodes of the liquid crystal shutters in sequence, and thereby prevent the molecular arrangement in the liquid crystals from being tilted in the field-free state.
Now description will be given of how conventional liquid crystal shutters are driven by the drive section in the illuminating device, with reference to FIG. 1.
FIG. 1 is an excerpt from drawings attached to Japanese Patent Application No. 2004-070317. FIG. 1 shows a timing relationship between an encoder signal ENC and liquid-crystal drive signal when the illuminating device mounted on the printer according to Japanese Patent Application No. 2004-070317 emits a round of three colored lights—R, G, and B—each time it receives a pulse. Incidentally, FIG. 1 shows an ideal encoder signal ENC which is a design center value.
The encoder signal ENC in FIG. 1 consists of a pulse train of pulses P1, P2, etc. These pulses P1, P2, etc. have ideal pulse intervals which are design center values. Incidentally, in FIG. 1, write times TR, TG, and TB for writing R, G, and B, respectively, according to the ideal pulse intervals are maximum values of exposure time needed to perform a round of writes.
The illuminating device of the printer according to Japanese Patent Application No. 2004-070317 emits a round of three colored lights—R, G, and B—each time it receives a pulse. Specifically, as shown in FIG. 1, it emits the R-colored light for the time TR starting at a rising edge of the pulse P1, the G-colored light for the time TG, and the B-colored light for the time TB.
In so doing, as shown in Part (c) of FIG. 1, the drive section in the illuminating device alternately applies voltages of positive (+) and negative (−) polarities between the electrodes of the liquid crystals, forming AC fields rather than DC fields in liquid crystals, and thereby preventing the molecular arrangement in the liquid crystals from being tilted in the field-free state.
However, since this printer provides a wait period (although referred to as a blank period TBL in FIG. 1, hereinafter the blank period will be referred to as a wait period) after a round of irradiation with the colored lights to adjust start timing of next irradiation (rising edge of the write command pulse P2) and thereby accommodate load changes, if transport speed is decreased, the wait period will be increased considerably.
FIG. 2 is a diagram illustrating a drive condition when transport speed of an instant film sheet is decreased.
As shown in Part (c) of FIG. 2, if transport speed of an instant film sheet is decreased, a cycle period of encoder pulses is increased, increasing a wait period (TBL in Part (d) of FIG. 2) after a round of writes. If the wait period is increased in this way, when a liquid crystal shutter is driven after the wait period, it responds differently compared to before the wait period.
FIG. 3 is a diagram showing how a response of a liquid crystal shutter varies with the length of a wait period. The vertical axis in FIG. 3 represents quantity of light passing through the liquid crystal shutter while the horizontal axis represents time. Incidentally, the liquid crystal shutter in FIG. 3 is a type which is closed when a voltage is applied between electrodes.
FIG. 3 shows a response waveform of the liquid crystal shutter when a stepwise voltage A is applied between the electrodes of liquid crystals to change the liquid crystal shutter from open state to closed state after a wait period of 4 msec and a response waveform of the liquid crystal shutter when a stepwise voltage A is applied between the electrodes of liquid crystals to change the liquid crystal shutter from open state to closed state after a wait period of 10 sec.
As shown in FIG. 3, a transient response waveform appearing during transition from open state to closed state tends to delay with increases in the wait period. If the response waveform appearing during transition from open state to closed state (or from closed state to open state) of the liquid crystal shutter changes in this way, any load change during transport of an instant film sheet will cause changes to the amount of exposure, resulting in color irregularities in a color image on the instant film sheet.
Also, although it is possible to put liquid crystals in the same open state (or closed state) by applying a voltage of either positive or negative polarity between electrodes, it is not possible to produce a precisely identical molecular arrangement in the liquid crystals using voltages of positive polarity and negative polarity. Referring to FIG. 1, although the drive section in the illuminating device alternately applies voltages of positive (+) and negative (−) polarities to produce an AC field in the liquid crystals, since there are an odd number (3) of light emitters, positive and negative voltages are alternately applied every other cycle in relation to each colored light.
Thus, due to physical properties in liquid crystals, there is a difference between the molecular arrangement resulting from application of a positive voltage and molecular arrangement resulting from application of a negative voltage in relation to each colored light. This can cause differences in the quantity of light passing through the liquid crystals.
This in turn can cause variations in the amount of exposure among write lines while the instant film sheet is being transported, resulting in color irregularities in a color image on the instant film sheet.
In this way, if the wait period until the next write is increased or voltages of different polarities are applied alternately to drive liquid crystals for each colored light, there can be color irregularities in a color image on the instant film sheet.